1. Field of the Invention
The present invention relates generally to gas turbine engines and, more specifically, to an improved cowl damping structure for use in the combustion chamber of such an engine.
2. Description of the Related Art
In an annular-type combustor of a gas turbine engine, pressurized air from the compressor is directed by guide vanes over the inner and outer liners of the combustion chamber, or combustor, to provide a cooling effect.
As shown in FIG. 1, a typical combustor 10 includes a combustion chamber 12 of generally annular configuration, as defined by an outer liner 14 and an inner line 16 of the chamber 12, each of the liners 14 and 16 being of a generally cylindrical configuration throughout at least a portion of the axial extent thereof, relatively to a central axis, or line ("C/L"), of the combustor 10 and thus of the gas turbine engine in general. The outer and inner cowls 18 and 20 are assembled with the chamber 12 by connecting their respective trailing edges 27a and 27b to the outer and inner liners 14 and 16, respectively, illustratively by bolts 28a and 28b and associated nuts. The leading edges 26a and 26b of the cowls 18 and 20 are thereby positioned in the vicinity of the fuel nozzles 22 and define therebetween a generally annular opening whereby compressed air is directed by guide vanes 24 through and around the cowls 18 and 20.
The cowls 18 and 20 accordingly are subjected to a very hostile environment, being impacted by chaotic perturbations in the impinging compressed air flow from the compressor and which in turn produce mechanical vibration of the cowls. Vibration resulting from these normal and unavoidable, adverse operating conditions produces high cycle fatigue of the cowls 18 and 20 and thus a life-shortening failure mechanism. Thus, vibration damping techniques have been developed to reduce the deleterious and life-shortening effects of such vibration.
One reasonably effective, prior art vibration damping technique, shown in FIG. 2(A) illustratively for the leading edge 26a, is to roll the fore end 18a of the sheet metal cowl 18 around and thereby partially encase a continuous, solid core wire 28; this structure produces a torsional frictional force between the contiguous, inner surface of the fore end 18a and the outer surface of the wire 28 and provides friction damping of the vibration.
Over long term exposure to the harsh operating conditions of the combustor, however, the wire-damped cowls are subject to the typical wear problems associated with friction (i.e., static part) damping. As shown in FIG. 2(A), the accumulated effects of wear result in the production of gradually increasing gaps 28a and 28b between the initially engaged contact surfaces. The frictional wear initially produces thinning of the wire 28 and/or the fore end 18a, followed by wire impact loading which alters the encased relationship, opening a further gap 28c (FIG. 2(B), the cumulative effects not only degrading the intended level of friction damping but also leading to shortened life and thus requiring more frequent replacement of the cowls is desired. Component testing of combustor cowls shows that the output response over a frequency range of new cowls varies significantly, the variation being attributable to manufacturing tolerances, required for reproducibility, in forming the leading edge 26a. Data from field cowls show a much higher output response than for new cowls, a result indicative of the degradation of the damping characteristic of the rolled wire leading edge as a function of the time of use. Inspection of damping wires from failed field parts has revealed wear of the respective contact areas of the damping wire and the rolled sheet metal.
Thus, a continuing need exists for a combustor cowl having means for damping vibrations which occur during normal operating conditions.